Anharmonic vibrations of the carboxyl group in acetic acid on TiO2: implications for adsorption mode assignment in dye-sensitized solar cells

We compute frequencies of vibrations of the carboxyl group in acetic acid adsorbed on the anatase (101) surface of TiO2 in two monodentate and the bidentate bridging configurations relevant for the adsorption of dyes on TiO2 in dye-sensitized solar cells (DSSCs). The ability to assign these vibrations and determine the adsorption configurations is critical for the design of DSSCs. Anharmonicity and coupling of four or five modes are taken into account by using a new version of the method of Manzhos and Carrington that computes vibrational spectra directly from discrete ab initio data, bypassing the construction of a potential energy surface, and using parameterized basis functions and rectangular collocation. We show that the method enables a routine analysis of anharmonic vibrations of practical importance in large systems. A sub-cm(-1) accuracy is achieved by using as few as 70 basis functions and 500 single-point energies. The calculations are doable on a desktop computer. This is the first time vibrational spectra for different adsorption sites of an organic molecule have been computed and compared without neglecting anharmonicity and coupling of the attaching group.